The invention relates to a process for the thermal treatment of meal-form raw materials, especially in the production of cement clinker from raw meal which is preheated in at least one heat exchanger train traversed by the exhaust gas of a rotary tubular kiln, calcined in a precalcining stage, and burned into cement clinker in the sintering zone of the rotary tubular kiln, which is cooled in a downstream-engaged cooler, in which process in the pre-calcination stage supplied with fuel, the exhaust gas stream from the rotary tubular kiln, and possibly an exhaust gas stream (tertiary air) of the clinker cooler are used separately or in common for the pre-calcination of the raw meal.
Processes of the type described, known, for example from EP-B-0 497 937, must be capable of producing, in the pre-calcination stage equipped with second-firings upstream-engaged to the rotary tubular kiln, a high-quality calcined raw meal, before introduction into the rotary tubular kiln, and to produce cement clinker with the lowest possible total energy requirement. Simultaneously the builders and operators of cement clinker production lines are confronted with ever sharper demand for low emission values of noxious substances such as CO and NOx.
For the reduction of the harmful substance NOx, it is a known practice to burn fuel substoichiometrically in the rotary kiln exhaust gas riser line, i.e., with oxygen insufficiency, for the purpose of creating a CO-containing reduction zone for the reduction of the harmful substance NOx which has been formed especially by high-temperature combustion in the rotary tubular kiln (thermal NOx). The CO not consumed in the NOx reduction zone of the rotary kiln exhaust gas channel, as well as solid-fuel particles at first not burned in the precalinator, are after-burned by the oxygen of a tertiary air stream introduced from the clinker cooler, this residual burning-out being favored by the swan""s neck-form flow deflection of the suspension in the re-calcining stage, especially if a turbulence chamber or mixing chamber is also arranged in the region of the flow deflection.
Despite these means, promoting burning-out of the residual substance in the pre-calcination stage it is not, however, entirely excluded in that there can occur cases in which CO thrusts or CO strands can pass with the exhaust gas into the cyclone fluidized gas heat exchanger system, especially with fluctuating solid-fuel dosing to the calcinator as well as with use of co-called secondary fuels, when, as known for example from EP-B-0 439 825, in the cement clinker production process whole scrap tires are disposed of, which should burn in the product inflow zone of the rotary tubular kiln with oxygen excess which, contained in excess secondary air, must be drawn through the entire rotary tubular kiln. Through the thrust-wise scrap-tire burning, undesired CO thrusts occurring there are virtually unavoidable.
Aside from the disadvantage of the incomplete fuel utilization, CO thrusts and unburned hydrocarbons in the exhaust gas of a cement clinker production line endanger, however, the safe operation of an exhaust gas dust remover in the form of an electrostatic dust separator (electrofilter), for which reason such dust separators are equipped with CO exhaust gas analyzers which shut off the electrofilter on occurrence of a CO thrust when an upper CO limit value is exceeded. In order to avoid such switch-offs, as well as CO and unburned hydrocarbons emissions, which would be brought about by local air deficiencies by reason of local unmixed fuel and air, and/or by non-homogeneous air or fuel dosing, cement clinker production lines have been operated by regulating intervention on the suction blower of the total exhaust gas, as a rule with an increased total-oxygen excess or air excess, for example with an air ratio xcex of 1.3 instead of xcex being, for example, 1.1 to 1.2, which through increased fuel and current costs endangers the economical operation of the cement clinker production line and which has a negative effect on the Nox emissions.
Underlying the invention is the problem of creating a process for the economical production of cement clinker in a cement clinker production line with raw meal preheater, a precalcination stage, a rotary tubular kiln, and a clinker cooler, in which system the total oxygen surplus necessary for all combustion points, i.e., the ratio of total oxygen to total fuel, often designated as xcex-value, lies as low as possible, without the possibility that even with use of unevenly dosed secondary fuels, undesirably high peaks and emissions of CO and/or unburned hydrocarbons occur.
In the process of the invention it is proposed, in the exhaust gas path of the precalcination stage of the cement clinker production installation, to measure continuously the content in CO and/or (unburned) hydrocarbons and/or NOx and/or other combustion products (CO2, O2, H2O) and, in dependence on this measurement value, perform a regulating intervention on the amount of the oxidation agent and/or catalyst to be sent into the precalcination stage, in such manner that, with rising CO and/or unburned hydrocarbons content, the injected amount of oxidation agent and/or catalyst is increased, and vice versa. If in the measuring an increase of the CO and/or unburned hydrocarbon content is ascertained, for example on the occasion of an above-described CO thrust occurring, then upon exceeding a predetermined upper limit value, for example 1.6 to 1.8% of CO in the area of the precalcination stage in which the residual burning-out as well as the residual calcination is to occur, the oxidation agent and/or the catalyst is/are injected for the acceleration of the chemical oxidation reaction process, in which case the injected amount is governed according to the requisite CO and/or unburned hydrocarbons decomposition, until a lower limit value of, for example 1.0% of CO is measured.
The injection time point, as well as the injected amount of oxidation agent and/or catalyst, can be determined not only from the exhaust gas analysis, but also by a tendency value which is calculated by a computer from the actual relations of fuel to oxygen determined at each and all combustion points of the cement clinker production line. In any case, the measurement values or computing values should be available without time lag after as short as possible a measuring time, in order to keep the consumption of oxidation agent and/or catalyst as low as possible.
As an oxidation agent accelerating the chemical reaction course of the oxidation in the cement clinker production line, there can be used air, air enriched with O2, and/or pure oxygen, and/or a substance from the peroxide group, for example hydrogen peroxide, and/or a substance from the perborate group, for example sodium perborate, and/or from the permanganate group, for example potassium permanganate, and/or from the nitrate group, for example potassium nitrate, and/or from the halogenates, for example potassium perchlorate, and/or oxides and oxo-acids of the halogens, for example perchloric acid, and/or nitric acid, and/or nitrotoluenes, such acid-containing compounds relatively easily giving off their oxygen, especially on heating. As a catalyst there can be used fine granular substances that further the combustion reaction and are usable in the cement clinker production process, such as, for example, certain cement raw meals and/or raw meal components and/or related-group elements and their compounds, such as, for example, iron oxide or its metallorganic compounds.
With the process of the invention there are achieved at least the throwing advantages:
The cement clinker production is economical because the combustion processes occur in the cement clinker production line, especially also in the precalcination stage with as low as possible an oxygen excess. The exhaust gas losses are minimized; the avoidance of CO peaks in the exhaust gas leads to less frequent switchings-off of electrofilters. Finally, in the cement clinker production process of the invention there can also be used difficultly dosable secondary fuels and/or lumpy fuels such as, for example, whole used tires without danger; i.e., even severe fluctuations in the type of fuel supplied is controllable. From the process of the invention also cement clinker burning installations can profit when a fuel gasification reactor is used in which secondary fuels with fluctuating fuel qualities are gasified. Finally, through the regulated injection of the oxidation agent and/or catalyst used, the substances used there can simultaneously be beneficially disposed of.